#include<stdio.h>
#include<math.h>
int main()
{
    //freopen("in.txt","r",stdin);
    double x1,y1,x2,y2;
    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
    // printf("%.8f",(x2-x1)*(x2-x1) + (y2-y1) * (y2-y1));
    printf("%.8f",sqrt( (x2-x1)*(x2-x1) + (y2-y1) * (y2-y1) ) );
}